Update MimeSniffingResourceHandler tests to use MockResourceLoader.

content/browser/loader/resource_controller.h

Index: content/browser/loader/resource_controller.h
diff --git a/content/browser/loader/resource_controller.h b/content/browser/loader/resource_controller.h
index f21a2039b0f6983c1e69ff4fca6e01fcd6cb7d43..98b2b72ff7069828335e815faae3dbc3a8a3c323 100644
--- a/content/browser/loader/resource_controller.h
+++ b/content/browser/loader/resource_controller.h
@@ -19,6 +19,10 @@ class CONTENT_EXPORT ResourceController {
   virtual void Cancel() = 0;
   virtual void CancelAndIgnore() = 0;
   virtual void CancelWithError(int error_code) = 0;
+
+  // Resumes the request. May only be called if the request was previously
+  // deferred. Guaranteed not to call back into the ResourceHandler, or destroy
+  // it, synchronously.
   virtual void Resume() = 0;
 
  protected: